This invention relates to a driving method for a solid-state imaging device provided with a blooming prevention structure.
In solid-state imaging devices which are used widely such as CCD type solid-state imaging devices and MOS type solid-state imaging devices, each pixel photoelectrically converts incident light into charge and accumulates the charge, and then the accumulated charge is read out as signal charge.
A solid-state imaging device of the type mentioned is normally provided with a blooming prevention structure. Blooming is a phenomenon wherein, when strong light is irradiated upon a certain pixel and a large amount of charge is generated in the pixel, the charge overflows from the pixel and enters pixels around the pixel thereby to cause an image at a high light location to look in an expanded condition.
The blooming prevention structure is a structure which causes charge generated by photoelectric conversion by each pixel section to overflow so that the accumulated charge of the pixel section may remain at an overflow level which is a given maximum accumulation charge amount to prevent occurrence of blooming.
However, if the maximum accumulated charge amount is determined in this manner, then the dynamic range is limited by it. Consequently, if a large amount of light which causes charge which exceeds the overflow level to be generated is inputted to some pixel sections, then all of the levels of output signals of the pixel sections exhibit an equal value, resulting in loss of light amount information.
In order to moderate this problem, a method has been proposed wherein the overflow level is varied from a low level to a high level midway in a charge accumulation period of each pixel so that the light amount to output signal characteristic may have a KNEE characteristic.
A MOS type solid state imaging device to which the method just described is applied is disclosed in Japanese Patent Publication Application No. 4-32589, wherein a MOS type solid state imaging device provided with a longitudinal overflow drain structure is constructed such that the overflow level is raised by varying the potential of a p-well with respect to an n-type substrate.
However, with the method just described, since the overflow level rises simultaneously at all pixels of a same solid-state imaging device, the start and end timings of charge accumulation must be adjusted among all of the pixels, and use of a lens shutter is essentially required.
The fact that the start and end timings of charge accumulation must be adjusted among all pixels signifies that the sensitivity drops because it is not possible to perform, while reading out of signals from a certain line is proceeding, charge accumulation in another line. Further, the fact that a lens shutter is essentially required narrows applications of the solid-state imaging device.